A liquid crystal display is consisted of, among many other things, a liquid crystal layer. The alignment behavior of this liquid crystal layer determines most of the optical properties of the liquid crystal display. This alignment is determined by the alignment layers in contact with the liquid crystal layer itself. The most common form of this alignment layer is a thin film of polyimide deposited on the substrate, which is usually glass. The preparation conditions of these top and bottom alignment layers provide an alignment direction as well as a pretilt angle for the liquid crystal molecules near the surface of the liquid crystal layer.
The pretilt angles achievable with common polyimides are predetermined and cannot be varied. In particular, it is very difficult to obtain large values of θ(0) from 10°-80°. There are needs to make liquid crystal cells with large pretilt angles in the region near 40-60 degrees. Many new types of liquid crystal displays including bistable displays and fast response liquid crystal displays can be made only if high pretilt angles are available.
Traditionally, the best way to obtain high pretilt angles controllably is by oblique evaporation of SiOx in a vacuum. This is a well-known art and has been discussed in many open publications (see David Armitage, J Appl Phys, vol 51, p 2552, 1980). Another well-known art is the use of molecular Langmuir-Bloggett films. All of these methods are, however, impractical for mass production.
More recently, a new method of ion beam alignment has been disclosed by Chaudhari et al. in U.S. Pat. No. 6,195,146. Many different pretilt angles can be made similar to SiOx evaporation. Additionally, several inventions have been disclosed recently to address the same issue of large pretilt angles. Harada et al. in U.S. Pat. No. 5,744,203 disclose a new alignment material that can achieve high pretilt by simple rubbing. Brosig et al. in U.S. Pat. No. 5,172,255 teach a method of achieving high pretilt angle in a homogeneous polyimide by rubbing the same surface twice in opposite directions. It was said that by varying the rubbing strength of the second rub, various pretilt angles can be achieved. Resnikov et al. Kim et al. and Gibbons et al. in U.S. Pat. Nos. 6,633,355, 5,882,238 and 5,856,430 respectively disclose means of achieving high pretilt angle by the method of photo alignment. These methods are different in the particular material of the alignment layer used, and the difference in the light illumination geometry. It is claimed that the photon dosage will vary the pretilt angle achieved ultimately. The detailed physics of the photoalignment technique is still not clear.
Several inventions have been disclosed making use of co-polymers and polymer blends as alignment layers. Mizushima et al discloses a co-polymer in U.S. Pat. Nos. 5,612,450 and 5,756,649, wherein the pretilt angle can be changed by radiation. Nishikawa et al discloses a polymer blend wherein the residual voltage and image retention can be reduced in U.S. Pat. Nos. 5,698,135 and 5,969,055. Nakajima et al in U.S. Pat. No. 5,119,221 describes a co-polymer where different functional groups are contained within the polymer. The variation of the functional group ratio can provide varying pretilt angles. In U.S. Pat. No. 6,731,362, Park et al discloses a polymer blend of polyimide and poly-cinnamate wherein the pretilt angle can be controlled to within 5 degrees.
Accordingly, it is an object of the present invention to provide a novel alignment layer capable of achieving high pretilt angles in a liquid crystal cell. These novel alignment layers consist of nano-structures of at least two types of alignment materials at close proximity to each other. The nano-structure can take on many shapes and can be in the form of nano-domains or nano-networks. The pretilt angle can be controlled by changing the nano-structure of the alignment layer and the relative proportion of the different alignment materials. It is also an object of the present invention to provide a novel method of producing such an alignment layer for a liquid crystal layer in a liquid crystal cell.